In recent years, various types of transmission formats have been reviewed for the standardization of Ethernet at 100 Gbps (gigabits/second). An attractive, spectrally efficient modulation format is DQPSK. And utilizing the two polarization modes supported by an optical fiber (polarization multiplexing) allows to further double the spectral efficiency. The DQPSK modulated signal lights are signals for transferring information by phase changes (four values of 0, π/2, π and 3π/2) between two symbols adjacent to each other. A signal light (to be referred to as a DQPSK polarization multiplexed signal light hereunder) which is obtained by polarization multiplexing a set of DQPSK modulated signal lights of which polarization states are different from each other, is capable of decreasing a baud rate, and also, has favorable polarization mode dispersion (PMD) tolerance and favorable chromatic dispersion resistance.
FIG. 6 shows a typical configuration example of an optical reception apparatus compatible with a DQPSK polarization multiplexing format. In this optical reception apparatus, a DQPSK polarization multiplexed signal light transmitted over a transmission path 101 of an optical communication system is input to a polarization beam splitter 103 via a polarization controller (PC) 102, to be split into a horizontally polarized signal light SH and a vertically polarized signal light SV. The horizontally polarized signal light SH is branched into two by an optical coupler (CPL) 104H, and the vertically polarized signal light SV is branched into two by an optical coupler (CPL) 104V. The branched lights SHI and SHQ of the optical coupler 104H are supplied to a substrate 105H, and the branched lights SVI and SVQ of the optical coupler 104V are supplied to a substrate 105V. On each of the substrates 105H and 105V, there is formed a set of delay interferometers corresponding to I branch and Q branch, and each of the delay interferometers is provided with an optical delay element with a delay equivalent to the duration of one symbol in the optical communication system. Further, an optical phase difference between the arms of each delay interferometer is set at “π/4” in I branch and at “−π/4” in Q branch. Two output terminals of each delay interferometer on the substrate 105H are connected to balanced optical detectors (O/E) 106HI and 106HQ, and also, two output terminals of each delay interferometer on the substrate 105V are connected to balanced optical detector (O/E) 106VI and 106VQ. Then, signals photo-electrically converted in the respective balanced optical detectors 106HI, 106HQ, 106VI and 106VQ are processed in a reception circuit 107, so that reception data DH obtained by demodulating the horizontally polarized DQPSK modulated signal light and reception data DV obtained by demodulating the vertically polarized DQPSK modulated signal light are regenerated.
Further, in the above optical reception apparatus, it is very important that the optical phase difference between the arms of each delay interferometer is exactly set at “π/4” on the substrates 105H and “−π/4” on the substrate 105V. If a deviation occurs in the optical phase difference, the signal can degraded beyond an allowable threshold. Therefore, the phase error is monitored by each of control circuits 108H and 108V based on output signals from the reception circuit 107, to perform a feedback control for optimizing the temperature and the like of each delay interferometer so that an optical phase is held at a target value (refer to Japanese Unexamined Patent Publications No. 2007-20138 and No. 2007-201939).
However, the optical reception apparatus compatible with the DQPSK polarization multiplexing format as shown in FIG. 6 problematically grows in size. Namely, the optical reception apparatus compatible with the DQPSK polarization multiplexing format needs a configuration twice as large as a DQPSK modulated signal light reception apparatus which does not perform the polarization multiplexing. In particular, each substrate formed with a set of delay interferometers is in a large size, since each delay interferometer performs the optical demultiplexing and the optical multiplexing. Accordingly, it is a problem how an implementing space for each delay interferometer is ensured within the apparatus. Further, since two substrates for delay interferometers are needed, the electric power, which is used by a device for regulating the temperature of each delay interferometer (for example, a heater, a Peltier unit or the like) when the optical phase difference is feedback controlled, becomes twice the electric power used for the case where the polarization multiplexing is not performed. Therefore, there is also a problem of an increase of power consumption. Furthermore, since the temperature of the delay interferometer is controlled by an analog circuit, characteristic variations in circuit parts have large effects, and accordingly, the precise circuit designing and the precise implementation are necessary. However, since the delay interferometer being a control object becomes twice, the characteristic variations in the parts used for the circuit corresponding to each delay interferometer need to become smaller, and accordingly, there is a problem in that the apparatus cost is increased.